Liquid crystal electrooptic devices such as flat panel displays rely on active alignment, or control, of the orientation of the liquid crystal molecules when no field is applied. A parameter of a liquid crystal structure, such as director orientation or smectic layer structure, may be said to be actively aligned if alignment layers induce a preferred configuration on the parameter and if when the preferred configuration is perturbed, the alignment layers exert a restoring force or torque.
There are a number of different conventional methods for controlling the orientation of the liquid crystals in the absence of a field. For example, in a twisted nematic display, the liquid crystal orientation is anchored at the surfaces on each side of the device and aligned parallel to the surfaces using rubbed polymer layers where the rubbing directions are mutually orthogonal to produce a twisted liquid crystal configuration. There are a number of difficulties associated with this approach, mainly associated with the rubbing procedure that is needed to induce the orientation in the alignment layers.
More problematic are smectic liquid crystal displays, such as, for example, ferroelectric liquid crystals (FLCs), used for bi-stable displays or newer analog “thresholdless FLC” devices. For FLC display panels and other smectic LCDs, the structure of the smectic layers as well as the orientation of the director is an important parameter. For existing smectic LCDs, the smectic layers of the FLC must be aligned in a “bookshelf arrangement,” and this orientation of the FLC is produced using polymer alignment layers with special thermal histories.
In addition to the same problems caused by rubbing that occur in nematic displays, a major deficiency in this means of controlling the oriented state is that they are very susceptible to mechanical disruption, and alignment generally does not recover after having been perturbed by mechanical stress. However, for LCDs containing the more ordered smectic liquid crystal materials, the smectic layer structure is only passively aligned by cooling through the nematic to smectic phase transition, i.e., there is no uniquely specified periodicity in the interaction between the alignment layer and adjacent liquid crystal molecules defining the alignment which the smectic layers should adopt. Thus, if this alignment is disturbed in the smectic phase, there is no force acting to restore the original alignment. Accordingly, a small mechanical shock can disrupt the orientation state, causing orientational defects to form, which cannot be removed by any existing technology. So while smectic LCDs and, in particular, ferroelectric LCDs are strong contenders for use in high definition television (HDTV) displays, memory displays, and computer work stations, their poor resistance to mechanical shock currently limits commercial FLC devices to small sizes, typically less than a few centimeters on a side. There are known ways of reducing this problem, such as, for example, through the use of damped mountings and adhesive spacer techniques for fabrication of FLC panels. However, these techniques are not effective against all possible types of mechanical damage, such as a sudden impact or continuous pressure.
Several patents attempt to address the problems associated with the stability of conventional liquid crystal displays via various conventional mechanical alignment layer means. For example, JP 52 411 discloses an arrangement in which dichromatic molecules are bonded to an alignment layer. Liquid crystal molecules then align on the layer of dichromatic molecules. However, this method still has the problem of a weak alignment layer-liquid crystal layer interface. Meanwhile, EP 307 959, EP 604 921 and EP 451 820 all disclose various techniques for obtaining particular structures within ferroelectric liquid crystal layers which are intended to provide improved mechanical stability. However, the structures disclosed in the specifications are incompatible with high speed, high contrast addressing schemes and are therefore of very limited application. EP 635 749 discloses an adhesive spacer technique for the fabrication of FLC display panels so as to provide more resistance to mechanical damage. However, as described hereinbefore, techniques of this type are not effective against all possible types of mechanical damage. Also, EP 467 456 discloses the use of a liquid crystal gel layer as an alignment layer. However, this type of alignment layer is used merely to control the pre-tilt angle of the liquid crystal material in the cell and does not improve the mechanical stability.
A second method for aligning liquid crystals uses a phase-separated polymer to control alignment and provide mechanical stability, rather than an alignment layer. There are two general techniques, polymer-dispersed liquid crystals and polymer-stabilized liquid crystals. These systems function similar to alignment layers, in that the interactions between the liquid crystal molecules and the polymer occur only at the interface between the solid polymer and the liquid crystal. Typically, the polymer is synthesized in situ by photochemistry or thermally triggered crosslinking of monomer (or macromer) dissolved into the liquid crystal. As the molecular weight of the polymer grows, the system phase-separates into polymer rich, solid and liquid crystal rich, nematic or smectic phases. The nature of the liquid crystal orientation at the resulting liquid crystal polymer interfaces is typically controlled by the structure of the polymer or surface-active agents that are incorporated in the system. In some cases, the orientation direction is influenced using an applied electric or magnetic field during polymerization so that the resulting polymer provides a lasting memory of the orientation state. In this technique the alignment polymer is made anisotropic by applying a flow or an electric field, then after the desired orientation of the solvated monomer or prepolymer is generated, the polymer is transformed so that it provides a lasting memory of the orientation state, e.g., by photochemically or thermally-triggered cross-linking. These techniques do improve the mechanical stability of the liquid crystals.
For example, GB 2 274 652 discloses an arrangement in which a conventional low molar mass ferroelectric liquid crystal mixture is doped with a polymeric additive. However, while this arrangement is intended to improve mechanical stability, of ferroelectric liquid crystals it results in reduced switching speed for the electrooptic device.
Similarly, EP 586 014 discloses arrangements of a polymer network created by photoinitiated polymerization of an aligned liquid crystal containing monomer. However, while this arrangement does improve mechanical stability, it results in reduced switching speed for the electrooptic device.
Finally, S. H. Jin et al, “Alignment of Ferroelectric Liquid-crystal Molecules by Liquid-Crystalline Polymer,” SID 95 Digest, (1995) 536–539 discloses the use of a main chain thermotropic liquid crystal polymer as an alignment layer for an FLC cell. However, the liquid crystal alignment is obtained by conventional mechanical rubbing of this layer, the liquid crystal polymer being in its glassy phase at room temperature.
Accordingly, a need exists for an improved material for use in aligning liquid crystal electrooptic devices which reduces or eliminates the need for a separate alignment layer and which provides greater mechanical stabilization to a wide range of fast switching liquid crystal displays.